Two types of memory devices dominate in the information storage arena. These are rotating memory and solid state memory. Rotating memories, such as disk drives of the magnetic, magneto-optic and optical variety, store as much as one gigabyte of data but offer modest read/write times on the order of milliseconds. On the other hand, solid state memories such as DRAMs (dynamic random access memory), SRAMs (static RAM) and EEPROMs (electrically erasable programmable nonvolatile memory) offer much higher read/write speeds on the order of nanoseconds, yet are of far lower capacity, i.e. in the few megabit range. In terms of cost per bit, rotating memory is cheaper, but it is also much more mechanically unreliable.
Of great value would be a memory device combining the capacity of rotating memory with the speed, size, and reliability of solid state memory. With it, computers would take another quantum leap in performance and compactness as well as in reliability. Along this vein, ideas for optically storing huge amounts of data in three dimensional electro-optic crystals or optical holograms have been discussed; however, their execution is far from reality, and even if they are ever executed, it will be exceedingly difficult to shrink such devices to microchip-size because of the optical scanning required and the lower limit on bitsize established by the wavelength of the laser used.